Hydrocarbon waste stream purification processes using microporous materials having filtration and adsorption properties

ABSTRACT

The present invention is directed to methods of treating a hydrocarbon-containing waste stream to form a hydrocarbon-containing retentate and an aqueous permeate which is substantially free of hydrocarbon. The method includes passing the hydrocarbon-containing waste stream through a microporous membrane to yield the hydrocarbon-containing retentate and the aqueous permeate. The membrane comprises a substantially hydrophobic, polymeric matrix and substantially hydrophilic, finely divided, particulate filler distributed throughout the matrix. The polymeric matrix has pores with a volume average diameter less than 1.0 micron, and at least 50 percent of the pores have a mean diameter of less than 0.35 micron.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/193,902, filed Feb. 28, 2014, entitled “FLUID EMULSIONPURIFICATION PROCESSES USING MICROPOROUS MATERIALS HAVING FILTRATION ANDADSORPTION PROPERTIES”, incorporated by reference herein in itsentirety, which is a continuation-in-part of U.S. patent applicationSer. No. 13/599,221, filed Aug. 30, 2012, entitled “MICROPOROUS MATERIALHAVING FILTRATION AND ADSORPTION PROPERTIES AND THEIR USE IN FLUIDPURIFICATION PROCESSES”, incorporated by reference herein in itsentirety, and which in turn claims the benefit of U.S. ProvisionalPatent Application No. 61/555,500, filed on Nov. 4, 2011, incorporatedby reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to hydrocarbon-containing waste streampurification processes using microporous materials having filtration andadsorption properties.

BACKGROUND OF THE INVENTION

According to the Department of Energy, 21 billion gallons of co-producedwater are drawn up by oil and gas wells each year in the United States.Natural “oil” from a well is actually a multiphase fluid ofoil/water/gas. Generally, all three fluids are found in everyhydrocarbon well and well effluent.

Because of its value and because of environmental concerns, oil needs tobe separated from this effluent. This is usually done throughgravitational settling in large tanks, which requires capital andsignificant space that is not always available onsite. Gas is separatedeasily in a mechanical separator or by pressure reduction within storagecontainers. In the case of heavy oils and many emulsified fluid systems,the raw fluids are heated to change the density of the oil and water byheating off lighter ends and essentially agitating their molecularstructures so that these fluids can more easily separate. Water then isa byproduct.

While co-produced water is common, it is a hazardous waste that is veryburdensome to operators and must be disposed of, sometimes withdifficulty. Water that historically may have been sent down a creek orinto a wooden barrel now has to be piped into very large storagecontainers. Offshore, those containers are attached to topsides orhooked by hoses to older, refitted storage ships. This waste water isstored for days to separate oil from water. The resulting aqueousproduct still contains some oil and other possible contaminants. Notonly is this procedure costly, it is time consuming, risky, and energyconsumptive. In an offshore subsea environment, water storage containersare not feasible.

Current topsides water-oil separation processes are slow,environmentally unfriendly, and consume a large amount of energy inheating. The final residual water must be disposed and cannot bereleased directly into the ocean. Stored water often must be treated bychemicals to reduce phase surface tensions and induce separation,creating further risk from chemical handling and possible spills.Volumes of produced water offshore can be in the thousands of barrelsper day per well. Processing facilities generally are a large part of anongoing footprint. Co-produced water is frequently re-injected into thesubsurface because topsides storage is a non-trivial issue. Sufficientremoval of controversial chemicals and residual oil before any disposalis a process concerning regulators, such as the U.S. EnvironmentalProtection Agency (EPA).

In subsea operations, as noted above, there is simply no large volumewater storage option available to industry. Separation is ineffective: asignificant amount of water-in-oil remains in solution after subseaseparation, resulting in inefficient separation and ultimately in lowerhydrocarbon recoveries. More critically, oil-in water concentrations arehigh (in the range of 5 percent) because of a lack of sufficientresidence time, which will result in crude oil injection into disposalwells along with produced water. Consequently, the disposal zoneseventually plug and become ineffective for additional disposal.Additionally, the oil-in-water that is pumped into disposal zones islost product and revenue to both the operating companies and to theFederal government from royalties.

It would be desirable to provide a simple, inexpensive technique thatcan overcome the aforementioned deficiencies. Such a technique can savethe industry hundreds of millions of dollars in operating andmaintenance costs, and increase revenues besides saving space.Additionally, that method can eliminate many environmental concerns.

SUMMARY OF THE INVENTION

The present invention is directed to a method of treating ahydrocarbon-containing waste stream comprising greater than 0 up to 2percent by weight hydrocarbon to form an aqueous permeate which issubstantially free of hydrocarbon and a hydrocarbon-containingretentate, wherein the method comprises passing thehydrocarbon-containing waste stream through a microporous membrane toyield the aqueous permeate and the hydrocarbon-containing retentate. Themembrane comprises a substantially hydrophobic, polymeric matrix andsubstantially hydrophilic, finely divided, particulate fillerdistributed throughout said matrix. The polymeric matrix has pores witha volume average diameter of less than 1.0 micron, and at least 50percent of the pores have a mean diameter of less than 0.35 micron.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of the cartridge permeate rates of the experimentalcartridge (Example 8) and the comparative cartridge (CE-9) disclosedherein.

FIG. 2 is a plot of the oil concentration in the permeate of theexperimental cartridge (Example 8) and the comparative cartridge (CE-9)disclosed herein.

DETAILED DESCRIPTION OF THE INVENTION

Other than in any operating examples, or where otherwise indicated, allnumbers expressing quantities of ingredients, reaction conditions and soforth used in the specification and claims are to be understood as beingmodified in all instances by the term “about”. Accordingly, unlessindicated to the contrary, the numerical parameters set forth in thefollowing specification and attached claims are approximations that mayvary depending upon the desired properties to be obtained by the presentinvention. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should at least be construed in light of thenumber of reported significant digits and by applying ordinary roundingtechniques.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contain certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements.

Also, it should be understood that any numerical range recited herein isintended to include all sub-ranges subsumed therein. For example, arange of “1 to 10” is intended to include all sub-ranges between (andincluding) the recited minimum value of 1 and the recited maximum valueof 10, that is, having a minimum value equal to or greater than 1 and amaximum value of equal to or less than 10.

As used in this specification and the appended claims, the articles “a”,“an”, and “the” include plural referents, unless expressly andunequivocally limited to one referent.

The various embodiments and examples of the present invention aspresented herein are each understood to be non-limiting with respect tothe scope of the invention.

As used in the following description and claims, the following termshave the meanings indicated below.

By “polymer” is meant a polymer including homopolymers and copolymers,and oligomers. By “composite material” is meant a combination of two ormore differing materials.

As used herein, “formed from” denotes open, e.g., “comprising”, claimlanguage. As such, it is intended that a composition “formed from” alist of recited components be a composition comprising at least theserecited components, and can further comprise other, non-recitedcomponents, during the composition's formation.

As used herein, the term “polymeric inorganic material” means apolymeric material having a backbone repeat unit based on an element orelements other than carbon. For more information see James Mark et al.,Inorganic Polymers, Prentice Hall Polymer Science and EngineeringSeries, (1992) at page 5, which is specifically incorporated byreference herein. Moreover, as used herein, the term “polymeric organicmaterials” means synthetic polymeric materials, semisynthetic polymericmaterials, and natural polymeric materials, all of which have a backbonerepeat unit based on carbon.

An “organic material”, as used herein, means carbon-containing compoundswherein the carbon is typically bonded to itself and to hydrogen, andoften to other elements as well, and excludes binary compounds such asthe carbon oxides, the carbides, carbon disulfide, etc.; such ternarycompounds as the metallic cyanides, metallic carbonyls, phosgene,carbonyl sulfide, etc.; and carbon-containing ionic compounds such asmetallic carbonates, for example calcium carbonate and sodium carbonate.See R. Lewis, Sr., Hawley's Condensed Chemical Dictionary, (12th Ed.1993) at pages 761-762, and M. Silberberg, Chemistry The MolecularNature of Matter and Change (1996) at page 586, which are specificallyincorporated by reference herein.

As used herein, the term “inorganic material” means any material that isnot an organic material.

As used herein, a “thermoplastic” material is a material that softenswhen exposed to heat and returns to its original condition when cooledto room temperature. As used herein, a “thermoset” material is amaterial that solidifies or “sets” irreversibly when heated or otherwisecured.

As used herein, “microporous material” or “microporous sheet material”means a material having a network of interconnecting pores, wherein, ona coating-free, printing ink-free, impregnant-free, and pre-bondingbasis, the pores have a volume average diameter (i.e., mean pore size)ranging from 0.001 to 1.0 micrometer, and constitute at least 5 percentby volume of the material as discussed herein below.

By “plastomer” is meant a polymer exhibiting both plastic andelastomeric properties.

By the term “emulsion” is meant a colloidal suspension of two liquidphases in which droplets of one liquid are suspended within anotherliquid. In most embodiments of the present invention, one liquid is anaqueous liquid. Such emulsions may be unstable or optionally stabilizedby surfactants or other emulsion stabilizers that are known in the art.

As noted above, the present invention is directed to a method oftreating a hydrocarbon-containing waste stream to form ahydrocarbon-containing retentate and an aqueous permeate which issubstantially free of hydrocarbon. The method comprises passing thehydrocarbon-containing waste stream through a microporous membrane toyield the hydrocarbon-containing retentate and the aqueous permeate. Themembrane comprises a substantially hydrophobic, polymeric matrix andsubstantially hydrophilic, finely divided, particulate fillerdistributed throughout said matrix, wherein the polymeric matrix haspores with a volume average diameter of less than 1.0 micron, and atleast 25 percent of the pores have a mean diameter of less than 0.3micron.

As mentioned above, the membrane comprises a substantially hydrophobic,polymeric matrix (discussed herein below) and substantially hydrophilic,finely divided, particulate filler distributed throughout the matrix.

-   -   The membranes suitable for use in the method of the present        invention typically include a microporous material. Suitable        microporous membranes generally comprise:    -   (a) a polyolefin matrix material present in an amount of at        least 2 percent by weight,    -   (b) hydrophilic, finely divided, particulate, substantially        water-insoluble filler, e.g., any of those described herein        below, distributed throughout the matrix, the filler        constituting from about 10 percent to about 90 percent by weight        of the microporous material, wherein the weight ratio of filler        to polyolefin is greater than 0.3; and    -   (c) at least 35 percent by volume of a network of        interconnecting pores communicating throughout the microporous        material. The microporous material generally is prepared by the        following method:        -   (i) mixing the polyolefin matrix material(s) (a), filler            (b), and a processing plasticizer until a substantially            uniform mixture is obtained;        -   (ii) introducing the mixture, optionally with additional            processing plasticizer, into a heated barrel of a screw            extruder and extruding the mixture through a sheeting die to            form a continuous sheet;        -   (iii) forwarding the continuous sheet formed by the die to a            pair of heated calender rolls acting cooperatively to form            continuous sheet of lesser thickness than the continuous            sheet exiting from the die;        -   (iv) optionally stretching the continuous sheet in at least            one stretching direction above the elastic limit, wherein            the stretching occurs during or immediately after step (ii)            and/or step (iii) but prior to step (v);        -   (v) passing the sheet to a first extraction zone where the            processing plasticizer is substantially removed by            extraction with an organic liquid;        -   (vi) passing the continuous sheet to a second extraction            zone where residual organic extraction liquid is            substantially removed by steam and/or water;        -   (vii) passing the continuous sheet through a dryer for            substantial removal of residual water and remaining residual            organic extraction liquid; and        -   (viii) optionally stretching the continuous sheet in at            least one stretching direction above the elastic limit,            wherein the stretching occurs during or after step (v), step            (vi), and/or step (vii) to form a microporous material.

It should be noted that while the polymeric matrix material may beheated and melted in an extruder as noted above in the preparation ofthe membrane sheet, it is unsintered; i.e., by ‘sintering’ is meant astep which causes the individual particles of material, for examplepolymer or resin, to adhere together in a solid porous matrix withoutthe need for a separately introduced binder, while retaining theirindividual identity as discreet particles to a substantial extent onheating. Sintering may be conducted on polymeric particles by heating inan oven at a temperature such as 150° C. for a time to allow foradhesion of particles to each other, such as at least 1 hour. Incontrast, the polymeric matrix material(s) used in the membranes in theprocess of the present invention undergoes a melt such that retention ofindividual polymeric particle identity does not occur in the preparationof the membranes used in the present invention.

Microporous materials used in the membranes may comprise a polyolefinmatrix. The polyolefin matrix is present in the microporous material inan amount of at least 2 percent by weight. Polyolefins are polymersderived from at least one ethylenically unsaturated monomer. In certainembodiments of the present invention, the matrix comprises a plastomer.For example, the matrix may comprise a plastomer derived from butene,hexene, and/or octene. Suitable plastomers are available from ExxonMobilChemical under the tradename “EXACT”.

The matrix can comprise a different polymer derived from at least oneethylenically unsaturated monomer, which may be used in place of or incombination with the plastomer. Examples include polymers derived fromethylene, propylene, and/or butene, such as polyethylene, polypropylene,and polybutene. High density and/or ultrahigh molecular weightpolyolefins, such as high density polyethylene, are also suitable.

The polyolefin matrix can comprise a copolymer of ethylene and butene.

Non-limiting examples of ultrahigh molecular weight (UHMW) polyolefincan include essentially linear UHMW polyethylene or polypropylene.Inasmuch as UHMW polyolefins are not thermoset polymers having aninfinite molecular weight, they are technically classified asthermoplastic materials.

The ultrahigh molecular weight polypropylene can comprise essentiallylinear ultrahigh molecular weight isotactic polypropylene. Often, thedegree of isotacticity of such polymer is at least 95 percent, e.g., atleast 98 percent.

While there is no particular restriction on the upper limit of theintrinsic viscosity of the UHMW polyethylene, in one non-limitingexample, the intrinsic viscosity can range from 18 to 39deciliters/gram, e.g., from 18 to 32 deciliters/gram. While there is noparticular restriction on the upper limit of the intrinsic viscosity ofthe UHMW polypropylene, in one non-limiting example, the intrinsicviscosity can range from 6 to 18 deciliters/gram, e.g., from 7 to 16deciliters/gram.

For purposes of the present invention, intrinsic viscosity is determinedby extrapolating to zero concentration the reduced viscosities or theinherent viscosities of several dilute solutions of the UHMW polyolefinwhere the solvent is freshly distilled decahydronaphthalene to which 0.2percent by weight, 3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid,neopentanetetrayl ester [CAS Registry No. 6683-19-8] has been added. Thereduced viscosities or the inherent viscosities of the UHMW polyolefinare ascertained from relative viscosities obtained at 135° C. using anUbbelohde No. 1 viscometer in accordance with the general procedures ofASTM D 4020-81, except that several dilute solutions of differingconcentration are employed.

The nominal molecular weight of UHMW polyethylene is empirically relatedto the intrinsic viscosity of the polymer in accordance with thefollowing equation:M=5.37×10⁴[{acute over (η)}]^(1.37)wherein M is the nominal molecular weight and [{acute over (η)}] is theintrinsic viscosity of the UHMW polyethylene expressed indeciliters/gram. Similarly, the nominal molecular weight of UHMWpolypropylene is empirically related to the intrinsic viscosity of thepolymer according to the following equation:M=8.88×104[{acute over (η)}]^(1.25)wherein M is the nominal molecular weight and [{acute over (η)}] is theintrinsic viscosity of the UHMW polypropylene expressed indeciliters/gram.

A mixture of substantially linear ultrahigh molecular weightpolyethylene and lower molecular weight polyethylene can be used. Incertain embodiments, the UHMW polyethylene has an intrinsic viscosity ofat least 10 deciliters/gram, and the lower molecular weight polyethylenehas an ASTM D 1238-86 Condition E melt index of less than 50 grams/10minutes, e.g., less than 25 grams/10 minutes, such as less than 15grams/10 minutes, and an ASTM D 1238-86 Condition F melt index of atleast 0.1 gram/10 minutes, e.g., at least 0.5 gram/10 minutes, such asat least 1.0 gram/10 minutes. The amount of UHMW polyethylene used (asweight percent) in this embodiment is described in column 1, line 52 tocolumn 2, line 18 of U.S. Pat. No. 5,196,262, which disclosure isincorporated herein by reference. More particularly, the weight percentof UHMW polyethylene used is described in relation to FIG. 6 of U.S.Pat. No. 5,196,262; namely, with reference to the polygons ABCDEF, GHCIor JHCK of FIG. 6, which Figure is incorporated herein by reference.

The nominal molecular weight of the lower molecular weight polyethylene(LMWPE) is lower than that of the UHMW polyethylene. LMWPE is athermoplastic material and many different types are known. One method ofclassification is by density, expressed in grams/cubic centimeter androunded to the nearest thousandth, in accordance with ASTM D 1248-84(Reapproved 1989). Non-limiting examples of the densities of LMWPE arefound in the following Table A.

TABLE A Type Abbreviation Density, g/cm³ Low Density LDPE 0.910-0.925Polyethylene Medium Density MDPE 0.926-0.940 Polyethylene High DensityHDPE 0.941-0.965 Polyethylene

Any or all of the polyethylenes listed in Table A above may be used asthe LMWPE in the matrix of the microporous material. HDPE may be usedbecause it can be more linear than MDPE or LDPE. Processes for makingthe various LMWPE's are well known and well documented. They include thehigh pressure process, the Phillips Petroleum Company process, theStandard Oil Company (Indiana) process, and the Ziegler process. TheASTM D 1238-86 Condition E (that is, 190° C. and 2.16 kilogram load)melt index of the LMWPE is less than about 50 grams/10 minutes. Oftenthe Condition E melt index is less than about 25 grams/10 minutes. TheCondition E melt index can be less than about 15 grams/10 minutes. TheASTM D 1238-86 Condition F (that is, 190° C. and 21.6 kilogram load)melt index of the LMWPE is at least 0.1 gram/10 minutes. In many cases,the Condition F melt index is at least 0.5 gram/10 minutes, such as atleast 1.0 gram/10 minutes.

The UHMWPE and the LMWPE may together constitute at least 65 percent byweight, e.g., at least 85 percent by weight, of the polyolefin polymerof the microporous material. Also, the UHMWPE and LMWPE together mayconstitute substantially 100 percent by weight of the polyolefin polymerof the microporous material.

The microporous material can comprise a polyolefin comprising ultrahighmolecular weight polyethylene, ultrahigh molecular weight polypropylene,high density polyethylene, high density polypropylene, or mixturesthereof.

If desired, other thermoplastic organic polymers also may be present inthe matrix of the microporous material provided that their presence doesnot materially affect the properties of the microporous materialsubstrate in an adverse manner. The amount of the other thermoplasticpolymer which may be present depends upon the nature of such polymer. Ingeneral, a greater amount of other thermoplastic organic polymer may beused if the molecular structure contains little branching, few long sidechains, and few bulky side groups, than when there is a large amount ofbranching, many long side chains, or many bulky side groups.Non-limiting examples of thermoplastic organic polymers that optionallymay be present in the matrix of the microporous material include lowdensity polyethylene, high density polyethylene,poly(tetrafluoroethylene), polypropylene, copolymers of ethylene andpropylene, copolymers of ethylene and acrylic acid, and copolymers ofethylene and methacrylic acid. If desired, all or a portion of thecarboxyl groups of carboxyl-containing copolymers can be neutralizedwith sodium, zinc, or the like. Generally, the microporous materialcomprises at least 70 percent by weight of UHMW polyolefin, based on theweight of the matrix. In a non-limiting embodiment, the above-describedother thermoplastic organic polymer are substantially absent from thematrix of the microporous material.

As previously mentioned, the membranes of the present invention furthercomprise substantially hydrophilic, finely divided, particulate fillerdistributed throughout the matrix. By “substantially hydrophilic” ismeant that the fillers have polar properties and have a tendency tointeract with, but are insoluble in, water and other polar substances.

Suitable fillers can include organic fillers such as hydrophilicpolymers, hydrophilic microspheres, hydrophilic biopolymers, and thelike. Non-limiting examples of suitable inorganic fillers can includesiliceous fillers and non-siliceous fillers. Such fillers can includeparticles of silica, titanium oxide, iron oxide, calcium oxide, copperoxide, zinc oxide, antimony oxide, zirconium oxide, magnesium oxide,alumina, molybdenum disulfide, zinc sulfide, barium sulfate, strontiumsulfate, calcium carbonate, magnesium carbonate, and magnesiumhydroxide. In a particular embodiment, the finely divided, particulatefiller comprises an inorganic filler material selected from the groupconsisting of silica, alumina, calcium oxide, zinc oxide, magnesiumoxide, titanium oxide, zirconium oxide, and mixtures thereof.

In particular embodiments of the present invention, the particulatefiller comprises silica particles, such as precipitated silicaparticles. It is important to distinguish precipitated silica fromsilica gel inasmuch as these different materials have differentproperties. Reference in this regard is made to R. K. Iler, TheChemistry of Silica, John Wiley & Sons, New York (1979). Library ofCongress Catalog No. QD 181.S6144, the entire disclosure of which isincorporate herein by reference. Note especially pages 15-29, 172-176,218-233, 364-365, 462-465, 554-564, and 578-579. Silica gel is usuallyproduced commercially at low pH by acidifying an aqueous solution of asoluble metal silicate, typically sodium silicate, with acid. The acidemployed is generally a strong mineral acid, such as sulfuric acid orhydrochloric acid, although carbon dioxide is sometimes used. Inasmuchas there is essentially no difference in density between gel phase andthe surrounding liquid phase while the viscosity is low, the gel phasedoes not settle out, that is to say, it does not precipitate. Silicagel, then, may be described as a non-precipitated, coherent, rigid,three-dimensional network of contiguous particles of colloidal amorphoussilica. The state of subdivision ranges from large, solid masses tosubmicroscopic particles, and the degree of hydration from almostanhydrous silica to soft gelatinous masses containing on the order of100 parts of water per part of silica by weight.

Precipitated silica is usually produced commercially by combining anaqueous solution of a soluble metal silicate, ordinarily alkali metalsilicate such as sodium silicate, and an acid so that colloidalparticles will grow in weakly alkaline solution and be coagulated by thealkali metal ions of the resulting soluble alkali metal salt. Variousacids may be used, including the mineral acids, but the preferred acidis carbon dioxide. In the absence of a coagulant, silica is notprecipitated from solution at any pH. The coagulant used to effectprecipitation may be the soluble alkali metal salt produced duringformation of the colloidal silica particles, it may be an electrolytesuch as a soluble inorganic or organic salt, or it may be a combinationof both.

Precipitated silica, then, may be described as precipitated aggregatesof ultimate particles of colloidal amorphous silica that have not at anypoint existed as macroscopic gel during the preparation. The sizes ofthe aggregates and the degree of hydration may vary widely.

Precipitated silica powders differ from silica gels that have beenpulverized and ordinarily having a more open structure, that is, ahigher specific pore volume. However, the specific surface area ofprecipitated silica as measured by the Brunauer, Emmet, Teller (BET)method using nitrogen as the adsorbate, is often lower than that ofsilica gel.

Many different precipitated silicas may be employed in the presentinvention, but the preferred precipitated silicas are those obtained byprecipitation from an aqueous solution of sodium silicate using asuitable acid such as sulfuric acid, hydrochloric acid, or carbondioxide. Such precipitated silicas are themselves known and processesfor producing them are described in detail in U.S. Pat. No. 2,940,830and in West German Offenlegungsschrift No. 35 45 615, the entiredisclosures of which are incorporated herein by reference, includingespecially the processes for making precipitated silicas and theproperties of the products.

The precipitated silicas used in the present invention can be producedby a process involving the following successive steps:

-   -   (a) an initial stock solution of aqueous alkali metal silicate        having the desired alkalinity is prepared and added to (or        prepared in) a reactor equipped with means for heating the        contents of the reactor,    -   (b) the initial stock solution within the reactor is heated to        the desired reaction temperature,    -   (c) acidifying agent and additional alkali metal silicate        solution are simultaneously added with agitation to the reactor        while maintaining the alkalinity value and temperature of the        contents of the reactor at the desired values,    -   (d) the addition of alkali metal silicate to the reactor is        stopped, and additional acidifying agent is added to adjust the        pH of the resulting suspension of precipitated silica to a        desired acid value,    -   (e) the precipitated silica in the reactor is separated from the        reaction mixture, washed to remove by-product salts, and    -   (f) dried to form the precipitated silica.

The washed silica solids are then dried using conventional dryingtechniques. Non-limiting examples of such techniques include ovendrying, vacuum oven drying, rotary dryers, spray drying, or spin flashdrying. Non-limiting examples of spray dryers include rotary atomizersand nozzle spray dryers. Spray drying can be carried out using anysuitable type of atomizer, in particular a turbine, nozzle,liquid-pressure or twin-fluid atomizer.

The washed silica solids may not be in a condition that is suitable forspray drying. For example, the washed silica solids may be too thick tobe spray dried. In one aspect of the above-described process, the washedsilica solids, e.g., the washed filter cake, are mixed with water toform a liquid suspension and the pH of the suspension adjusted, ifrequired, with dilute acid or dilute alkali, e.g., sodium hydroxide,from 6 to 7, e.g., 6.5, and then fed to the inlet nozzle of the spraydryer.

The temperature at which the silica is dried can vary widely but will bebelow the fusion temperature of the silica. Typically, the dryingtemperature will range from above 50° C. to less than 700° C., e.g.,from above 100° C., e.g., 200° C. to 500° C. In one aspect of theabove-described process, the silica solids are dried in a spray dryerhaving an inlet temperature of approximately 400° C. and an outlettemperature of approximately 105° C. The free water content of the driedsilica can vary, but is usually in the range of from approximately 1 to10 wt. %, e.g., from 4 to 7 wt. %. As used herein, the term “free water”means water that can be removed from the silica by heating it for 24hours at from 100° C. to 200° C., e.g., 105° C.

The dried silica also can be forwarded directly to a granulator where itis compacted and granulated to obtain a granular product. Dried silicacan also be subjected to conventional size reduction techniques, e.g.,as exemplified by grinding and pulverizing. Fluid energy milling usingair or superheated steam as the working fluid can also be used. Theprecipitated silica obtained is usually in the form of a powder.

Most often, the precipitated silica is rotary dried or spray dried.Rotary dried silica particles have been observed to demonstrate greaterstructural integrity than spray dried silica particles. They are lesslikely to break into smaller particles during extrusion and othersubsequent processing during production of the microporous material thanare spray dried particles. Particle size distribution of rotary driedparticles does not change as significantly as does that of spray driedparticles during processing. Spray dried silica particles are morefriable than rotary dried, often providing smaller particles duringprocessing. It is possible to use a spray dried silica of a particularparticle size such that the final particle size distribution in themembrane does not have a detrimental effect on water flux. In certainembodiments, the silica is reinforced; i.e., has a structural integritysuch that porosity is preserved after extrusion. More preferred is aprecipitated silica in which the initial number of silica particles andthe initial silica particle size distribution is mostly unchanged bystresses applied during membrane fabrication, such that a broad particlesize distribution is present in the finished membrane. Blends ofdifferent types of dried silica and different sizes of silica may beused to provide unique properties to the membrane. For example, a blendof silicas with a bimodal distribution of particle sizes may beparticularly suitable for certain separation processes. It is expectedthat external forces applied to silica of any type may be used toinfluence and tailor the particle size distribution, providing uniqueproperties to the final membrane.

The surface of the filler particles can be modified in any manner wellknown in the art, including, but not limited to, chemically orphysically changing its surface characteristics using techniques knownin the art. For example, the filler may be surface treated with ananti-fouling moiety such as polyethylene glycol, carboxybetaine,sulfobetaine and polymers thereof, mixed valence molecules, oligomersand polymers thereof, and mixtures thereof. Another embodiment may be ablend of fillers in which one has been treated with a positively chargedmoiety and the other has been treated with a negatively charged moiety.The filler also may be surface modified with functional groups such ascations or anions that allow for targeted removal of specificcontaminants in a fluid stream to be purified using the microporousmembrane. Untreated filler particles may also be used. Filler particlescoated with hydrophilic coatings can reduce fouling and may eliminatepre-wetting processing. Filler particles coated with hydrophobiccoatings also can reduce fouling and may aid degassing and venting of asystem.

Suitable filler particles, e.g., precipitated silica particles,typically have an average ultimate particle size of 1 to 100 nanometers.

The surface area of the filler particles, in particular, silicaparticles, is considered both external and internal surface area due tothe presence of the pores. Such surface area can have a significantimpact on performance. High surface area fillers are materials of verysmall particle size, materials having a high degree of porosity ormaterials exhibiting both characteristics. Usually, the surface area ofthe filler itself is in the range of from about 125 to about 700 squaremeters per gram (m²/g) as determined by the Brunauer, Emmett, Teller(BET) method according to ASTM C 819-77 using nitrogen as the adsorbatebut modified by outgassing the system and the sample for one hour at130° C. Often, the BET surface area is in the range of from about 190 to350 m²/g, more often, suitable filler particulates, in particular thesilica particulates, demonstrate a BET surface area of 351 to 700 m²/g.

The BET/CTAB quotient is the ratio of the overall filler particle (e.g.,precipitated silica) surface area including the surface area containedin pores only accessible to smaller molecules, such as nitrogen (BET),to the external surface area (CTAB). This ratio is typically referred toas a measure of microporosity. A high microporosity value, i.e., a highBET/CTAB quotient number, is a high proportion of internalsurface—accessible to the small nitrogen molecule (BET surface area) butnot to larger particles—to the external surface (CTAB).

It has been suggested that the structure, i.e., pores, formed within thefiller, e.g., precipitated silica, during its preparation can have animpact on performance. Two measurements of this structure are theBET/CTAB surface area ratio of the precipitated silica noted above, andthe relative breadth (γ) of the pore size distribution of theprecipitated silica. The relative breadth (γ) of pore size distributionis an indication of how broadly the pore sizes are distributed withinthe precipitated silica particle. The lower the γ value, the narrower isthe pore size distribution of the pores within the precipitated silicaparticle.

The silica CTAB values may be determined using a CTAB solution and thehereinafter described method. The analysis is performed using a Metrohm751 Titrino automatic titrator, equipped with a Metrohm Interchangeable“Snap-In” 50 milliliter buret and a Brinkmann Probe Colorimeter Model PC910 equipped with a 550 nm filter. In addition, a Mettler Toledo HB43 orequivalent is used to determine the 105° C. moisture loss of the silicaand a Fisher Scientific Centrific™ Centrifuge Model 225 may be used forseparating the silica and the residual CTAB solution. The excess CTABcan be determined by auto titration with a solution of Aerosol OT® untilmaximum turbidity is attained, which can be detected with the probecolorimeter. The maximum turbidity point is taken as corresponding to amillivolt reading of 150. Knowing the quantity of CTAB adsorbed for agiven weight of silica and the space occupied by the CTAB molecule, theexternal specific surface area of the silica is calculated and reportedas square meters per gram on a dry-weight basis.

Solutions required for testing and preparation include a buffer of pH9.6, cetyl [hexadecyl] trimethyl ammonium bromide (CTAB), dioctyl sodiumsulfosuccinate (Aerosol OT) and 1N sodium hydroxide. The buffer solutionof pH 9.6 can be prepared by dissolving 3.101 g of orthoboric acid (99%;Fisher Scientific, Inc., technical grade, crystalline) in a one-litervolumetric flask, containing 500 milliliters of deionized water and3.708 grams of potassium chloride solids (Fisher Scientific, Inc.,technical grade, crystalline). Using a buret, 36.85 milliliters of the1N sodium hydroxide solution was added. The solution is mixed anddiluted to volume.

The CTAB solution is prepared using 11.0 g±0.005 g of powdered CTAB(cetyl trimethyl ammonium bromide, also known as hexadecyl trimethylammonium bromide, Fisher Scientific Inc., technical grade) onto aweighing dish. The CTAB powder is transferred to a 2-liter beaker andthe weighing dish rinsed with deionized water. Approximately 700milliliters of the pH 9.6 buffer solution and 1000 milliliters ofdistilled or deionized water is added to the 2-liter beaker and stirredwith a magnetic stir bar. The beaker may be covered and stirred at roomtemperature until the CTAB powder is totally dissolved. The solution istransferred to a 2-liter volumetric flask, rinsing the beaker and stirbar with deionized water. The bubbles are allowed to dissipate and thesolution diluted to volume with deionized water. A large stir bar can beadded and the solution mixed on a magnetic stirrer for approximately 10hours. The CTAB solution can be used after 24 hours and for only 15days. The Aerosol OT® (dioctyl sodium sulfosuccinate, Fisher ScientificInc., 100% solid) solution may be prepared using 3.46 g±0.005 g, whichis placed onto a weighing dish. The Aerosol OT® on the weighing dish isrinsed into a 2-liter beaker, which contains about 1500 milliliters ofdeionized water and a large stir bar. The Aerosol OT® solution isdissolved and rinsed into a 2-liter volumetric flask. The solution isdiluted to the 2-liter volume mark in the volumetric flask. The AerosolOT® solution is allowed to age for a minimum of 12 days prior to use.The shelf life of the Aerosol OT® solution is 2 months from thepreparation date.

Prior to surface area sample preparation, the pH of the CTAB solutionshould be verified and adjusted as necessary to a pH of 9.6±0.1 using 1Nsodium hydroxide solution. For test calculations, a blank sample shouldbe prepared and analyzed. 5 milliliters of the CTAB solution arepipetted and 55 milliliters of deionized water added into a150-milliliter beaker and analyzed on a Metrohm 751 Titrino automatictitrator. The automatic titrator is programmed for determination of theblank and the samples with the following parameters: Measuring pointdensity=2, Signal drift=20, Equilibrium time=20 seconds, Start volume=0ml, Stop volume=35 ml, and Fixed endpoint=150 mV. The buret tip and thecolorimeter probe are placed just below the surface of the solution,positioned such that the tip and the photo probe path length arecompletely submerged. Both the tip and photo probe should be essentiallyequidistant from the bottom of the beaker and not touching one another.With minimum stirring (setting of 1 on the Metrohm 728 stirrer) thecolorimeter is set to 100% T prior to every blank and sampledetermination and titration initiated with the Aerosol OT® solution. Theend point can be recorded as the volume (ml) of titrant at 150 mV.

For test sample preparation, approximately 0.30 grams of powdered silicawas weighed into a 50-milliliter container containing a stir bar.Granulated silica samples were riffled (prior to grinding and weighing)to obtain a representative sub-sample. A coffee mill style grinder wasused to grind granulated materials. Then 30 milliliters of the pHadjusted CTAB solution was pipetted into the sample container containingthe 0.30 grams of powdered silica. The silica and CTAB solution was thenmixed on a stirrer for 35 minutes. When mixing was completed, the silicaand CTAB solution were centrifuged for 20 minutes to separate the silicaand excess CTAB solution. When centrifuging was completed, the CTABsolution was pipetted into a clean container minus the separated solids,referred to as the “centrifugate”. For sample analysis, 50 millilitersof deionized water was placed into a 150-milliliter beaker containing astir bar. Then 10 milliliters of the sample centrifugate was pipettedfor analysis into the same beaker. The sample was analyzed using thesame technique and programmed procedure as used for the blank solution.

For determination of the moisture content, approximately 0.2 grams ofsilica was weighed onto the Mettler Toledo HB43 while determining theCTAB value. The moisture analyzer was programmed to 105° C. with theshut-off 5 drying criteria. The moisture loss was recorded to thenearest+0.1%.

The external surface area is calculated using the following equation,

${{CTAB}\mspace{14mu}{Surface}\mspace{14mu}{Area}\mspace{14mu}{\left( {{dried}\mspace{14mu}{basis}} \right)\left\lbrack {m^{2}\text{/}g} \right\rbrack}} = \frac{\left( {{2\; V_{o}} - V} \right) \times (4774)}{\left( {V_{o}W} \right) \times \left( {100 - {Vol}} \right)}$wherein,

-   -   V_(o)=Volume in ml of Aerosol OT® used in the blank titration.    -   V=Volume in ml of Aerosol OT® used in the sample titration.    -   W=sample weight in grams.    -   Vol=% moisture loss (Vol represents “volatiles”).

Typically, the CTAB surface area of the silica particles used in thepresent invention ranges from 120 to 500 m²/g. Often, the silicademonstrates a CTAB surface area of 170-280 m²/g. More often, the silicademonstrates a CTAB surface area of 281-500 m²/g.

In certain embodiments of the present invention, the BET value of theprecipitated silica will be a value such that the quotient of the BETsurface area in square meters per gram to the CTAB surface area insquare meters per gram is equal to or greater than 1.0. Often, the BETto CTAB ratio is 1.0-1.5. More often, the BET to CTAB ratio is 1.5-2.0.

The BET surface area values reported in the examples of this applicationwere determined in accordance with the Brunauer-Emmet-Teller (BET)method in accordance with ASTM D 1993-03. The BET surface area can bedetermined by fitting five relative-pressure points from a nitrogensorption isotherm measurement made with a Micromeritics TriStar 3000™instrument. A flow Prep-060™ station provides heat and a continuous gasflow to prepare samples for analysis. Prior to nitrogen sorption, thesilica samples are dried by heating to a temperature of 160° C. inflowing nitrogen (P5 grade) for at least one (1) hour.

The filler particles can constitute from 10 to 90 percent by weight ofthe microporous material. For example, such filler particles canconstitute from 25 to 90 percent by weight of the microporous material,such as from 30 percent to 90 percent by weight of the microporousmaterial, or from 40 to 90 percent by weight of the microporousmaterial, or from 50 to 90 percent by weight of the microporousmaterial, and even from 60 percent to 90 percent by weight of themicroporous material. The filler is typically present in the microporousmaterial of the present invention in an amount of 25 percent to about 85percent by weight of the microporous material. Often, the weight ratioof filler to polyolefin in the microporous material is greater than 0.3,frequently 1.4 to 3.5:1. Alternatively, the weight ratio of filler topolyolefin in the microporous material may be greater than 4:1, forexample as high as 10:1.

The microporous material used in the membrane employed in the process ofthe present invention further comprises a network of interconnectingpores communicating throughout the microporous material. The pores havea volume average diameter less than 1.0 micron, such as a volume averagediameter ranging from 0.01 to 0.9 micron, or from 0.02 to 0.7 micron, orfrom 0.02 to 0.5 micron, or from 0.03 to 0.5 micron. Moreover, at least50 percent, such as at least 55 percent, or at least 60 percent, or atleast 70 percent of the pores have a mean diameter of less than 0.35micron.

On an impregnant-free basis, such pores can comprise at least 15 percentby volume, e.g., from at least 20 to 95 percent by volume, or from atleast 25 to 95 percent by volume, or from 35 to 70 percent by volume ofthe microporous material. Often, the pores comprise at least 35 percentby volume, or even at least 45 percent by volume of the microporousmaterial. Such high porosity provides higher surface area throughout themicroporous material, which in turn facilitates removal of contaminantsfrom a fluid stream and higher flux rates of a fluid stream through themembrane.

As used herein and in the claims, the porosity (also known as voidvolume) of the microporous material, expressed as percent by volume, isdetermined according to the following equation:Porosity=100[1−d ₁ /d ₂]wherein d₁ is the density of the sample, which is determined from thesample weight and the sample volume as ascertained from measurements ofthe sample dimensions, and d₂ is the density of the solid portion of thesample, which is determined from the sample weight and the volume of thesolid portion of the sample. The volume of the solid portion of the sameis determined using a Quantachrome Stereopycnometer™ (QuantachromeCorp.) in accordance with the accompanying operating manual.

The volume average diameter, or mean pore size, of the pores of themicroporous material can be determined by mercury porosimetry using anAuto Pore III porosimeter (Micromeritics, Inc.) in accordance with theaccompanying operating manual.

The volume average pore radius for a single scan is automaticallydetermined by the porosimeter. In operating the porosimeter, a scan ismade in the high pressure range (from 138 kilopascals absolute to 227megapascals absolute). If approximately 2 percent or less of the totalintruded volume occurs at the low end (from 138 to 250 kilopascalsabsolute) of the high pressure range, the volume average pore diameteris taken as twice the volume average pore radius determined by theporosimeter. Otherwise, an additional scan is made in the low pressurerange (from 7 to 165 kilopascals absolute) and the volume average porediameter is calculated according to the equation:d=2[v ₁ r ₁ /w ₁ +v ₂ r ₂ /w ₂ ]/[v ₁ /w ₁ +v ₂ /w ₂]wherein d is the volume average pore diameter, v₁ is the total volume ofmercury intruded in the high pressure range, v₂ is the total volume ofmercury intruded in the low pressure range, r₁ is the volume averagepore radius determined from the high pressure scan, r₂ is the volumeaverage pore radius determined from the low pressure scan, w₁ is theweight of the sample subjected to the high pressure scan, and w₂ is theweight of the sample subjected to the low pressure scan. The volumeaverage diameter of the pores is typically less than 1.0 micrometer(micron), and can be, for example, in the range of from 0.001 to 0.7micron, such as 0.02 to 0.7 micron, or from 0.02 to 0.5 micron, or from0.3 to 0.7 micron. Also, as previously mentioned, at least 50 percent ofthe pores can have a mean diameter of at least 0.35 micron. In thecourse of determining the volume average pore diameter using the aboveprocedure, the maximum pore radius detected is sometimes noted. This istaken from the low pressure range scan, if run; otherwise it is takenfrom the high pressure range scan. The maximum pore diameter is twicethe maximum pore radius. Inasmuch as some production or treatment steps,e.g., coating processes, printing processes, impregnation processesand/or bonding processes, can result in the filling of at least some ofthe pores of the microporous material, and since some of these processesirreversibly compress the microporous material, the parameters inrespect of porosity, volume average diameter of the pores, and maximumpore diameter are determined for the microporous material prior to theapplication of one or more of such production or treatment steps.

Porosity also can be measured using a Gurley Densometer, model 4340,manufactured by GPI Gurley Precision Instruments of Troy, N.Y. Theporosity values reported are a measure of the rate of air flow through asample or it's resistance to an air flow through the sample. The unit ofmeasure for this method is a “Gurley second” and represents the time inseconds to pass 100 cc of air through a 1 inch square area using apressure differential of 4.88 inches of water. Lower values equate toless air flow resistance (more air is allowed to pass freely). Forpurposes of the present invention, the measurements are completed usingthe procedure listed in the manual for MODEL 4340 Automatic Densometer.

In certain embodiments of the present invention, to prepare themicroporous materials, filler, polymer powder (polyolefin polymer),processing plasticizer, and minor amounts of lubricant and antioxidantare mixed until a substantially uniform mixture is obtained. The weightratio of filler to polymer powder employed in forming the mixture isessentially the same as that of the microporous material substrate to beproduced. The mixture, together with additional processing plasticizer,is introduced to the heated barrel of a screw extruder. Attached to theextruder is a die, such as a sheeting die, to form the desired endshape.

In an exemplary manufacturing process, the material is formed into asheet or film, and a continuous sheet or film formed by a die isforwarded to a pair of heated calender rolls acting cooperatively toform a continuous sheet of lesser thickness than the continuous sheetexiting from the die. The final thickness may depend on the desiredend-use application. The microporous material may have a thicknessranging from 0.7 to 18 mil (17.8 to 457.2 microns) and demonstrates abubble point of 1 to 80 psi based on ethanol.

In certain embodiments, the sheet exiting the calender rolls is thenstretched in at least one stretching direction above the elastic limit.Stretching may alternatively take place during or after exiting from thesheeting die or during calendering, or multiple times, but it istypically done after extraction. Stretched microporous materialsubstrate may be produced by stretching the intermediate or finalproduct in at least one stretching direction above the elastic limit.Usually, the stretch ratio is at least about 1.1. In many cases, thestretch ratio is at least about 1.3, such as at least about 1.5.Frequently, the stretch ratio is in the range of from about 1.5 to about15. Often, the stretch ratio is in the range of from about 1.7 to about10. Preferably, the stretch ratio is in the range of from about 2 toabout 6.

The temperatures at which stretching is accomplished may vary widely.Stretching may be accomplished at about ambient room temperature, butusually elevated temperatures are employed. The intermediate product maybe heated by any of a wide variety of techniques prior to, during,and/or after stretching. Examples of these techniques include radiativeheating such as that provided by electrically heated or gas firedinfrared heaters, convective heating such as that provided byrecirculating hot air, and conductive heating such as that provided bycontact with heated rolls. The temperatures which are measured fortemperature control purposes may vary according to the apparatus usedand personal preference. For example, temperature-measuring devices maybe placed to ascertain the temperatures of the surfaces of infraredheaters, the interiors of infrared heaters, the air temperatures ofpoints between the infrared heaters and the intermediate product, thetemperatures of circulating hot air at points within the apparatus, thetemperature of hot air entering or leaving the apparatus, thetemperatures of the surfaces of rolls used in the stretching process,the temperature of heat transfer fluid entering or leaving such rolls,or film surface temperatures. In general, the temperature ortemperatures are controlled such that the intermediate product isstretched about evenly so that the variations, if any, in film thicknessof the stretched microporous material are within acceptable limits andso that the amount of stretched microporous material outside of thoselimits is acceptably low. It will be apparent that the temperatures usedfor control purposes may or may not be close to those of theintermediate product itself since they depend upon the nature of theapparatus used, the locations of the temperature-measuring devices, andthe identities of the substances or objects whose temperatures are beingmeasured.

In view of the locations of the heating devices and the line speedsusually employed during stretching, gradients of varying temperaturesmay or may not be present through the thickness of the intermediateproduct. Also, because of such line speeds, it is impracticable tomeasure these temperature gradients. The presence of gradients ofvarying temperatures, when they occur, makes it unreasonable to refer toa singular film temperature. Accordingly, film surface temperatures,which can be measured, are best used for characterizing the thermalcondition of the intermediate product.

These are ordinarily about the same across the width of the intermediateproduct during stretching although they may be intentionally varied, asfor example, to compensate for intermediate product having awedge-shaped cross section across the sheet. Film surface temperaturesalong the length of the sheet may be about the same or they may bedifferent during stretching.

The film surface temperatures at which stretching is accomplished mayvary widely, but in general they are such that the intermediate productis stretched about evenly, as explained above. In most cases, the filmsurface temperatures during stretching are in the range of from about20° C. to about 220° C. Often, such temperatures are in the range offrom about 50° C. to about 200° C. From about 75° C. to about 180° C. ispreferred.

Stretching may be accomplished in a single step or a plurality of stepsas desired. For example, when the intermediate product is to bestretched in a single direction (uniaxial stretching), the stretchingmay be accomplished by a single stretching step or a sequence ofstretching steps until the desired final stretch ratio is attained.Similarly, when the intermediate product is to be stretched in twodirections (biaxial stretching), the stretching can be conducted by asingle biaxial stretching step or a sequence of biaxial stretching stepsuntil the desired final stretch ratios are attained. Biaxial stretchingmay also be accomplished by a sequence of one of more uniaxialstretching steps in one direction and one or more uniaxial stretchingsteps in another direction. Biaxial stretching steps where theintermediate product is stretched simultaneously in two directions anduniaxial stretching steps may be conducted in sequence in any order.Stretching in more than two directions is within contemplation. It maybe seen that the various permutations of steps are quite numerous. Othersteps, such as cooling, heating, sintering, annealing, reeling,unreeling, and the like, may optionally be included in the overallprocess as desired.

Various types of stretching apparatus are well known and may be used toaccomplish stretching of the intermediate product. Uniaxial stretchingis usually accomplished by stretching between two rollers, wherein thesecond or downstream roller rotates at a greater peripheral speed thanthe first or upstream roller. Uniaxial stretching can also beaccomplished on a standard tentering machine. Biaxial stretching may beaccomplished by simultaneously stretching in two different directions ona tentering machine. More commonly, however, biaxial stretching isaccomplished by first uniaxially stretching between two differentiallyrotating rollers as described above, followed by either uniaxiallystretching in a different direction using a tentering machine or bybiaxially stretching using a tentering machine. The most common type ofbiaxial stretching is where the two stretching directions areapproximately at right angles to each other. In most cases wherecontinuous sheet is being stretched, one stretching direction is atleast approximately parallel to the long axis of the sheet (machinedirection) and the other stretching direction is at least approximatelyperpendicular to the machine direction and is in the plane of the sheet(transverse direction).

Stretching the sheets prior to or after extraction of the processingplasticizer allows for larger pore sizes than in microporous materialsconventionally processed, thus making the microporous materialparticularly suitable for use in the microfiltration membranes of thepresent invention. It is also believed that stretching of the sheetsprior to extraction of the processing plasticizer minimizes thermalshrinkage after processing.

The product passes to a first extraction zone where the processingplasticizer is substantially removed by extraction with an organicliquid which is a good solvent for the processing plasticizer, a poorsolvent for the organic polymer, and more volatile than the processingplasticizer. Usually, but not necessarily, both the processingplasticizer and the organic extraction liquid are substantiallyimmiscible with water. The product then passes to a second extractionzone where the residual organic extraction liquid is substantiallyremoved by steam and/or water. The product is then passed through aforced air dryer for substantial removal of residual water and remainingresidual organic extraction liquid. From the dryer, the microporousmaterial may be passed to a take-up roll, when it is in the form of asheet.

The processing plasticizer has little solvating effect on thethermoplastic organic polymer at 60° C., only a moderate solvatingeffect at elevated temperatures on the order of about 100° C., and asignificant solvating effect at elevated temperatures on the order ofabout 200° C. It is a liquid at room temperature and usually it isprocessing oil such as paraffinic oil, naphthenic oil, or aromatic oil.Suitable processing oils include those meeting the requirements of ASTMD 2226-82, Types 103 and 104. Those oils which have a pour point of lessthan 22° C., or less than 10° C., according to ASTM D 97-66 (reapproved1978) are used most often. Examples of suitable oils include Shellflex®412 and Shellflex® 371 oil (Shell Oil Co.) which are solvent refined andhydrotreated oils derived from naphthenic crude. It is expected thatother materials, including the phthalate ester plasticizers such asdibutyl phthalate, bis(2-ethylhexyl) phthalate, diisodecyl phthalate,dicyclohexyl phthalate, butyl benzyl phthalate, and ditridecyl phthalatewill function satisfactorily as processing plasticizers.

There are many organic extraction liquids that can be used. Examples ofsuitable organic extraction liquids include 1,1,2-trichloroethylene,perchloroethylene, 1,2-dichloroethane, 1,1,1-trichloroethane,1,1,2-trichloroethane, methylene chloride, chloroform, isopropylalcohol, diethyl ether, and acetone.

In the above-described process for producing microporous materialsubstrate, extrusion and calendering are facilitated when the fillercarries much of the processing plasticizer. The capacity of the fillerparticles to absorb and hold the processing plasticizer is a function ofthe surface area of the filler. Therefore, the filler typically has ahigh surface area as discussed above. Inasmuch as it is desirable toessentially retain the filler in the microporous material substrate, thefiller should be substantially insoluble in the processing plasticizerand substantially insoluble in the organic extraction liquid whenmicroporous material substrate is produced by the above process.

The residual processing plasticizer content is usually less than 15percent by weight of the resulting microporous material and this may bereduced even further to levels, such as less than 5 percent by weight,by additional extractions using the same or a different organicextraction liquid.

The resulting microporous materials may be further processed dependingon the desired application. For example, a hydrophilic or hydrophobiccoating may be applied to the surface of the microporous material toadjust the surface energy of the material. Also, the microporousmaterial may be adhered to a support layer such as a fiberglass layer toprovide additional structural integrity, depending on the particular enduse. Additional optional stretching of the continuous sheet in at leastone stretching direction may also be done during or immediately afterany of the steps upon extrusion in step (ii). In the production of amicrofiltration membrane, typically the stretching step occurs afterextraction of the plasticizer.

The microporous materials prepared as described above are suitable foruse as the membranes in the processes of the present invention, and arecapable of removing particulates from a fluid stream ranging in sizefrom 0.05 to 1.5 microns. The membranes also serve to remove molecularcontaminants from a fluid stream by adsorption and/or by physicalrejection due to molecular size, allowing for separation of ahydrocarbon-containing waste stream into a hydrocarbon-containingretentate and an aqueous permeate.

The membranes may be in the form of a sheet or housed (i.e., as acomponent) in a filter assembly. Any suitable filter assembly known inthe art may be used, with the membrane described above used as theseparation medium. The membrane housed within the filter assembly may bein any practical configuration; it is typically configured to maximizesurface area contact with the fluid being treated, such as by pleating.Non-limiting examples of suitable filter assemblies include spiralcrossflow filters as described in U.S. Pat. No. 8,454,829; FIGS. 1-8 andcolumn 4, line 46 to column 8, line 47 of the reference are incorporatedherein by reference. The method of the present invention comprisescontacting a hydrocarbon-containing waste stream with the membrane,typically by passing the stream through the membrane, to form ahydrocarbon-containing retentate. The retentate has a higherconcentration of hydrocarbon than that of the hydrocarbon-containingwaste stream being treated. Also formed is an aqueous permeate which issubstantially free from hydrocarbon. As used herein with respect to theaqueous permeate produced by the method of the present invention, by“substantially free of hydrocarbon” is meant that the aqueous permeatecontains less than 50 parts per million (ppm) hydrocarbon, e.g., lessthan 45 ppm, or less than 40 ppm, or less than 30 ppm, or less than 25ppm, or less than 20 ppm hydrocarbon. The aqueous permeate produced bythe method of the present invention is sufficiently purified of free,dissolved or emulsified hydrocarbon.

The hydrocarbon-containing waste stream may be passed through severalsuch membranes in series; however, a single pass is sufficient to yieldthe purities noted above, thus rendering the process of the presentinvention superior to conventional separation processes that useconventional filters and membranes.

Hydrocarbon-containing waste streams to be treated by the process of thepresent invention contain both hydrocarbon and aqueous phases. Examplesinclude crude oil well effluents (both inland and offshore oilproduction effluents) such as fracking effluents and conventional oildrilling effluents. Other crude oil well effluents such as wastewaterstreams from oil production including oil and gas drilling muds,drilling additives, polyacrylamide polymers, and other hydrocarbon basedwell additives and oil production brines are also suitable fluidemulsions. The method of the invention can be used to treathydrocarbon-containing waste streams comprising any percent by weighthydrocarbon and/or water. For example, the process of the invention canbe used to treat waste streams comprising hydrocarbon and water, withthe hydrocarbon comprising at least 0.1 percent by weight of the wastestream, for example, at least 0.2 percent by weight, or at least 0.5percent by weight, for example, at least 1 percent by weight, forexample, at least 2 percent by weight, for example, at least 5 percentby weight. For example, the process of the invention can be used totreat hydrocarbon-containing waste streams comprising greater than 0 upto 2 percent by weight hydrocarbon and at least 98 percent by weightwater. For example, the method of the present invention can be used inthe treatment of a hydrocarbon-containing waste stream comprising 0.1 to2.0 percent hydrocarbon, such as 0.1 to 1.5 percent hydrocarbon, or 0.1to 1.0 percent hydrocarbon, or 0.2 to 0.75 percent hydrocarbon.

For the purposes of the present invention, flux rates may be expressedin gallons/(ft²×psi×day) (GFD/psi). The fluid, hydrocarbon-containingwaste stream to be treated usually is passed through the membrane at aflux rate of 0.05 to 20 GFD/psi. Again, the waste stream to be treatedmay be passed through several membranes in series, but because of thehigh efficacy of the membranes used in the method of the presentinvention, a single pass typically is sufficient to achieve aqueouspermeate purities that are compliant with industry standards, describedquantitatively above. Moreover, in certain embodiments, thehydrocarbon-containing waste stream does not undergo any separationprocesses such as settling in one or more serial tanks, distillation, orcentrifugation prior to passing through the membrane. Preliminaryseparation processes are not necessary.

In the method of the present invention, the hydrocarbon-containing wastestream to be treated is contacted with the membrane described above. Theaqueous phase of the waste stream passes through the membrane forming anaqueous permeate. The hydrocarbon phase does not pass through themembrane and forms a hydrocarbon-containing retentate. Thehydrocarbon-containing retentate may be recirculated through themembrane at least once, either by itself for further purification orupon admixture with additional untreated hydrocarbon-containing wastestream feed.

A plurality of membranes configured in parallel filter assemblies may beused in the method of the present invention to achieve a desired flowrate, such as a flow rate of at least 125 gal/min. It has been observedthat minimal fouling of the membrane occurs during operation when usedto treat oil well effluents, in contrast to conventional separationtechnologies used for oil well effluents. The membranes used in theprocess of the present invention are significantly more resistant tofouling than conventional separation systems. Therefore, in certainembodiments of the present invention, the flow rate of thehydrocarbon-containing waste stream during the process is at least 125gal/min over at least 168 hours of service time of the microfiltrationmembrane.

It is particularly noteworthy that the membrane can demonstrate ahydrocarbon retention rate, and the retentate and aqueous permeateproduced by the method of the present invention have a purity thatgenerally are independent of (i) flux rate of the aqueous phase passingthrough the membrane and (ii) the pore size of the membrane. While notintending to be bound by theory, it is believed that surfaceinteractions between the various phases of the fluid waste stream andthe respective hydrophilic and hydrophobic components of the membranecontribute to the hydrocarbon retention rate of the membrane and thepurities of the aqueous permeate produced by the method of the presentinvention. The present invention is more particularly described in thefollowing examples which are intended to be illustrative only, sincenumerous modifications and variations therein will be apparent to thoseskilled in the art.

EXAMPLES

Part I: Preparation of Stretched Sheet Microporous Materials

The microporous membranes used in the following examples were preparedin two steps. First, microporous membranes were prepared according toprocedures previously described. Microporous Membrane A was preparedaccording to Example 2 of US 2014/0069862. Microporous Membrane B wasprepared according to Example 3 of US 2014/0069862. Microporous MembraneC was prepared according to the procedure of Example 3 of US2014/0069862, using 1000 parts silica and 1330 parts TUFFLO® 6056(commercially available from PPC Lubricants).

The membranes described above were then stretched to produce membraneswith the desired properties and pore sizes. Stretching was conducted byParkinson Technologies, Inc. using the Marshall and Williams BiaxialOrientation Plastic Processing System. The Machine Direction Oriented(MDO) stretching of the materials, where indicated, was accomplished byheating the specified membrane material and stretching it in the machinedirection over a series of rollers maintained at the temperatures listedin Table 1.

After MDO stretching and where indicated, Transverse DirectionOrientation (TDO) stretching was conducted by heating the resultantsheets according to the temperature conditions listed in Table 1, andstretching in the transverse (or cross) direction on a tenter frame,consisting of two horizontal chain tracks, on which clip and chainassemblies held the material in place.

TABLE 1 Stretched Microporous Membrane Preparation Examples 1 2 3 4 CE-5Microporous Sheet Material A B B C B MDO Stretch roll (° C.) — 132 132132 132 Anneal roll (° C.) — 141 141 141 141 Cooling (° C.) — 25 25 2525 Slow draw — 10.4 10.4 10.4 10.4 speed (FPM) Fast Roll Speed — 35 3530 30 (FPM) TDO Stretch ratio 1 — 2 4 4 Preheat (° C.) 132 — 132 141 132Stretching (° C.) 132 — 132 141 132 Anneal (° C.) 132 — 132 141 132 Linespeed 30 — 35 30 35 (FPM)

Comparative Membrane Examples

CE-5 was a stretched membrane prepared in Table 1 above.

CE-6 was GE-Osmonics ULTRAFILIC® membrane, a treated polyacrylonitrilemembrane sold commercially by Sterlitech Corporation for oil/waterseparation.

CE-7 was HFM® 180, a polyvinylidene difluoride ultrafiltration membrane,commercially available from Sterlitech Corporation.

Part II: Membrane Materials and Physical Properties

The physical properties of the membrane materials referenced in theexamples and comparative examples are summarized in Table 2.

TABLE 2 Physical Properties of Membrane Materials Gurley Mean CumulativeThick- Poros- Bubble Pore pore Exam- ness¹ ity² Point³ MW Size³ number<0.35 ples (μm) (sec) (bar) Cut-off (μm) micron (%) 1 135 40 3.6 N/A0.15 100 2 200 150 4.5 N/A 0.10 100 3 80 180 4.8 N/A 0.08 100 4 70 251.7 N/A 0.36 67 CE-5 76 20 1.6 N/A 0.39 0 CE-6 198 854 N/A 20-50K 0.01⁴unknown CE-7 210 416 N/A 100K n/a unknown ¹Thickness was determined byusing an Ono Sokki thickness gauge EG-225. The thickness reported is anaverage of 9 measurements. ²Porosity was measured with a GurleyDensometer, model 4340, manufactured by GPI Gurley Precision Instrumentsof Troy, New York according to the operating instructions. ³ASTM F316-03was followed, using a Porometer 3G manufactured by QuantachromeInstruments. POROFEL ® Wetting Fluid, available from QuantachromeInstruments, was used to wet the membrane before testing. ⁴As reportedin product literaturePart III: Oil/Water Cross-Flow Separation Test

Water flux testing was carried out using a cross-flow test cellapparatus, Model CF-042 available from Sterlitech Corporation, WA, asdescribed in the examples below. Testing was performed on crudeoil/water preparations with 0.125%, 0.25%, 0.5%, and 0.75% by weight ofcrude oil content. The crude oil in water cross-flow test results foreach oil concentration are presented in Tables 4, 5, 6, 7, and 8respectively.

Part IIIA: Model Aqueous Crude Oil Preparations

For each oil concentration, 8 gallons of deionized water was chargedinto a feed tank and stirred with a 5-inch blade driven by a compressionair pressure of 30 psi. Designated amounts of the salts listed in Table3 below were slowly added to the feed tank. The water and the salts weremixed by stirring constantly for 30 min. until a homogenous solution wasformed.

TABLE 3 Salt Composition in Aqueous Preparations Salts Parts by WeightCa(NO₃)₂*4H₂O 3.92 MgSO₄ 0.15 NaCl 4.58 NaNO₃ 0.74 FeCl₃ (50%) 0.01K₂CO₃ 0.02 Sodium Dodecyl Sulfate (SDS) 0.01

Then designated amounts of Texas crude oil (“Texas Raw Crude Oil”,obtained from Texas Raw Crude, Midland, Tex.) were slowly added to thefeed tank to attain 0.125%, 0.25%, 0.50%, and 0.75% by weight of crudeoil content respectively. The preparation was maintained by continuousstirring with an air driven motor during the cross-flow separation testdescribed below.

Part IIIB: Cross-Flow Separation Test

The water flux testing on the microporous membranes was carried out withthe four concentrations of crude oil/water preparation described above.For each test, the membrane effective area was 42 cm². The apparatus wasplumed with 4 cells in parallel test lines. Each cell was equipped witha valve to turn the feed flow on or off and regulate the flow rate,which was set to 1.5 GPM (gallon per minute) in all tests. Pressuregauges were located at the inlet and outlet of the apparatus. Thepressure was maintained at 15 psi trans membrane pressure (TMP). Thetest apparatus was equipped with a temperature controller to maintainthe temperature 25±2° C. and the results were reported asgallons/(ft²×psi×day). i.e., 24 hours (GFD/psi). The permeate sample wascollected after 30 min. of testing. The oil content in the permeate wasmeasured following EPA method 413.2. The oil retention rate (R) wasdetermined using the following formula:R=100*(C_(feed)−C_(permeate))/C_(feed) wherein C_(feed) is theconcentration of oil in the feed stream and C_(permeate) is the oilconcentration in the permeate. The results for cross-flow separationtests at each crude oil concentration are shown in the following Tables4, 5, 6, and 7 respectively.

TABLE 4 0.125% Aqueous Oil Cross-flow Separation Oil Oil ConcentrationFlux Turbidity⁴ Retention in the Permeate Membrane (GFD/psi) (NTU) Rate(%) (ppm) 1 18.83 1 99.7 3.6 2 5.88 2 99.5 6.1 4 24.08 1 99.8 3.2 CE-516.83 4 98.8 15 CE-6 1.96 4 99.3 9.0 CE-7 5.60 9 96.6 43 ⁴Turbidity wasmeasured using a Model 2100AN Laboratory Turbidimeter purchased fromHach Company.

TABLE 5 0.25% Aqueous Oil Cross-flow Separation Oil Oil ConcentrationFlux Turbidity Retention in the Permeate Membrane (GFD/psi) (NTU) Rate(%) (ppm) 1 11.21 5 99.2 19 2 5.88 1 99.7 7.6 3 3.88 2 99.8 4.5 4 15.0 199.9 3.3 CE-5 15.69 5 98.8 29 CE-6 0.84 7 99.0 25 CE-7 2.69 6 96.4 89

TABLE 6 0.50% Aqueouse Oil Cross-flow Separation Oil Oil ConcentrationFlux Turbidity Retention in the Permeate Membrane (GFD/psi) (NTU) Rate(%) (ppm) 1 8.40 10 98.9 28 2 3.92 5 99.2 20 CE-5 12.33 20 98.6 35 CE-60.45 17 99.0 25 CE-7 0.50 16 96.2 95

TABLE 7 0.75% Aqueous Oil Cross-flow Separation Oil Oil ConcentrationFlux Turbidity Retention in the Permeate Membrane (GFD/psi) (NTU) Rate(%) (ppm) 1 5.60 13 99.4 42 2 2.80 7 99.4 45 CE-5 11.21 37 98.7 94 CE-60.22 N/A⁵ N/A⁵ N/A⁵ CE-7 0.28 N/A⁵ N/A⁵ N/A⁵ ⁵Insufficient permeatecollected.Part IV: Oil/Water Cartridge Separation Test

Water flux testing was also carried out using 8040 spiral woundcartridges fabricated with an experimental membrane and with apolyacrylonitrile membrane as described below. An 8040 cartridge wasfabricated with the membrane of Example 1, comprising an effectivemembrane active area of 275 ft², using a diamond feed spacer andthermoplastic netting outer wrap. This is presented as Example 8. ForComparative Example CE-9, a GE MW 8040C1066 cartridge was purchased fromLenntech By. This cartridge utilizes the ULTRAFILIC membranedemonstrated above as CE-6. Testing was performed on a crude oil/waterpreparation starting at 0.25% by weight of crude oil content.

Part IVA: Oil in Water Waste Stream Preparation

The aqueous oil preparation for the cartridge separation test wasprepared in the same proportions as described in Part IIIA to obtain 300gallons of solution and 0.25% by weight of crude oil content. Thepreparation was maintained by continuous stirring with an air drivenmotor during the cartridge separation test described below.

Part IVB: Cartridge Separation Test

The water flux testing on the microporous membranes was carried out withthe experimental (Example 8) and comparative (CE-9) cartridges describedabove. The feed flow inlet was controlled at 90-105 Gal/min, and thecartridge transmembrane pressure (TMP) was maintained between 8-12 psi.The permeate flux rate was recoded with time and reported as Gal; min.The test was run approximately 12 continuous hours at a time, shuttingdown overnight. During the rest, both the solution in the permeate andthe solution from the flow outlet flowed directly back into the300-gallon tank. An additional 340 g of oil was added to the tank every8 hours of service. The oil content in the permeate was measuredfollowing EPA method 413.2. The results for cartridge permeate rate andthe oil concentration in the permeate are shown in FIG. 1 and FIG. 2.

The data in Tables 4 through 7 demonstrate the ability of themicroporous membranes of the present invention to effect good removal ofoil from permeate while maintaining good flux rates compared tocommercially-available polyacrylonitrile and polyvinylidene difluoridemembranes. The robustness of the inventive membranes is furtherdemonstrated in commercial-type cartridge configurations in FIG. 1 andFIG. 2, where the commercial polyacrylonitrile cartridge CE-9 assemblyshows reduced flow and decreased effectiveness in removing oil from thepermeate over time compared to Example 8, the cartridge configured withthe membrane of Example 1.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the scope of the inventionas defined in the appended claims.

What is claimed is:
 1. A method of treating a hydrocarbon-containingwaste stream comprising greater than 0 up to 2 percent by weighthydrocarbon to form an aqueous permeate which is substantially free ofhydrocarbon and a hydrocarbon-containing retentate, wherein the methodcomprises passing the hydrocarbon-containing waste stream through amicroporous membrane to yield the aqueous permeate and thehydrocarbon-containing retentate, wherein the membrane comprises asubstantially hydrophobic, polymeric matrix and substantiallyhydrophilic, finely divided, particulate filler distributed throughoutsaid matrix, wherein the polymeric matrix has pores with a volumeaverage diameter less than 1.0 micron, and wherein at least 50 percentof the pores have a mean diameter of less than 0.35 micron.
 2. Themethod of claim 1, wherein the membrane comprises at least 35 percent byvolume of a network of interconnecting pores communicating throughoutthe membrane.
 3. The method of claim 1, wherein the membrane is in theform of a sheet.
 4. The method of claim 1, wherein the polymeric matrixhas pores with a volume average diameter ranging from 0.03 to 0.5micron.
 5. The method of claim 1, wherein the finely divided,particulate filler comprises an inorganic filler material selected fromthe group consisting of silica, alumina, calcium oxide, zinc oxide,magnesium oxide, titanium oxide, zirconium oxide, and mixtures thereof.6. The method of claim 5, wherein the finely divided, particulate fillercomprises silica.
 7. The method of claim 6, wherein the finely divided,particulate filler comprises precipitated silica present in thepolymeric matrix in an amount ranging from 10 to 90 percent by weight.8. The method of claim 1, wherein the polymeric matrix comprisespolyolefin.
 9. The method of claim 1, wherein the filler has beensurface modified with functional groups that react with or adsorb one ormore materials in the fluid stream.
 10. The method of claim 9, whereinthe filler has been surface modified with hydrophilic functional groups.11. The method of claim 1, wherein the hydrocarbon-containing wastestream comprises 0.1 to 1.0 percent by weight hydrocarbon.
 12. Themethod of claim 1, wherein the hydrocarbon-containing waste streamcomprises 0.2 to 0.75 percent by weight hydrocarbon.
 13. The method ofclaim 1, wherein the aqueous permeate contains less than 50 parts permillion hydrocarbon.
 14. The method of claim 1, wherein at least 55percent of the pores have a mean diameter of less than 0.35 micron. 15.The method of claim 1, wherein said membrane is housed in a filterassembly.
 16. The method of claim 15, wherein the filter assembly isselected from the group consisting of a pleated filter and a spiralcrossflow filter.